Diamond and Pearl-I remember that that game had some pretty difficult sections like Mt. Coronet and Victory Road. The Gym Leaders were also pretty difficult. On the other hand when it's been a long time since I played that game and when I first played it I was younger and I was very bad at Pokemon(DP was my first pokemon game).
Sun and Moon-Some Totem Pokemon like Lurantis were very difficult.
Emerald- Don't really remember the game well but the caves were pretty difficult, especially the ones where you needed Flash.
Omega Ruby and Alpha Sapphire-Cruised through this game
X and Y-Mega evolution and easy gyms made this game a joke.